The present invention relates to a field emission display (referred to as xe2x80x9cFEDxe2x80x9d hereinafter); and, more particularly, to a cell driving apparatus for achieving an advanced gray scale by adjusting an amount of current provided to a cathode.
Recently, a liquid crystal display (referred to as xe2x80x9cLCDxe2x80x9d hereinafter), which displays pictures by selectively intercepting optical beams emitted from an optical source, has been in the limelight as one of flat panel displays. The LCD is operated by two methods one of which is a passive matrix method and the other an active matrix method.
The passive matrix method stores image data on a pixel, which is defined by an intersection of two selected electrodes, by applying different voltages at an upper plate and a lower plate of the LCD, respectively. In case the LCD employs the passive matrix method, a compensation circuit is needed in order to improve image quality since one pixel can affect its surrounding pixels. As a result, a cell driving circuit of the LCD becomes complicated.
On the other hand, by employing the active matrix method, each pixel, having a cell transistor and a capacitor therein, in the LCD stores previous data until next data is inputted thereto. Accordingly, the image quality of the LCD can be improved and also the cell driving circuit can be simplified.
However, although the active matrix method can achieve an improved image quality and a simplicity of the cell driving circuit, there are drawbacks such as that a manufacturing process of the LCD becomes complex and that its productivity is decreased since a substantial amount of transistors and capacitors should be deposited on a crystal substrate of the LCD.
The LCD suffers from high power consumption since only part of light from the power is actually used in displaying the pictures. It is also difficult to generate the LCD of a large size. Furthermore, since the LCD uses tiny, sealed capsules which contain transparent liquid crystals, it has limitations such as sensitivity to temperature change in surrounding environment, weakness to pressure, and a low resolution.
To overcome the above drawbacks, a field emission display (FED) is proposed. The FED displays pictures in a similar manner used in a cathode-ray tube (CRT) which displays the pictures by using emitted electrons. However, the FED uses a cold electron emission unlike the CRT which uses a thermal electron emission.
The FED sets up for each pixel a field emission device which emits electrons and displays the pictures by using electrons which collide with an electrode having a fluorescent plate deposited thereon. Recently, such FED is in the limelight as a next generation flat panel display capable of overcoming the drawbacks of the LCD mentioned above.
The FED can integrate hundreds or thousands of field emission devices in order to produce one pixel. Referring to FIG. 1, each of the field emission devices constituting the pixel of the FED comprises a cathode 12 connected to a cathode electrode 10, a gate electrode 14 deposited on the cathode 12, and an anode 18 having a fluorescent plate 16 deposited on the back of the anode 18.
In the above, the fluorescent plate 16 generates a light corresponding to an amount of electrons colliding thereon so as to display the pictures.
The anode pulls the electrons emitted from the cathode 12 and is transparent thereby making it possible transmit the light through the fluorescent plate 16.
The cathode 12 has a conic structure as shown in FIG. 1 and emits electrons from its cone by an operating voltage derived from the cathode electrode 10.
The gate electrode 14 induces the emission of electrons from the cathode 12 by using a high-voltage which is less than a voltage provided to the anode 18 and the emitted electrons are directed to the cathode 12 having a higher voltage.
A cell driving method of the FED containing the above field emission devices can be a passive matrix method or an active matrix method. They, i.e., the two matrix methods are similar to those used in the LCD.
The passive matrix method generally drives a cell by using a difference between a gate voltage Vg provided to a gate line and a cathode voltage Vk applied to a cathode line. By using the above passive matrix method, full color can be readily achieved. However, since a rate of current-to-voltage of tips is non-linear and the tips are not uniformly deposited, it is difficult to control a level of current.
Although the passive matrix method outputs the cathode voltage Vk as a pulse pattern with a predetermined number of pulses while the gate voltage Vg is maintained at a high level thereby representing a gray level by using the number of pulses, it has a disadvantage of having a limitation in representing the gray scale.
Meanwhile, as the active matrix method, a method, which is described in U.S. Pat. No. 5,210,472, is noticed. By driving a cell by using the active matrix method disclosed in the above patent, there are advantages of minimizing crosstalk and addressing with a lower voltage.
According to the cell driving performed by the active matrix method, the gray scale is represented by a pulse width modulation (PWM), and, thus, it is difficult to achieve full color. Also, a transistor should be integrated on each cell and, thereafter, there exists a complexity in manufacturing processes and a high cost.
Therefore, in order to overcome the above disadvantages, a cell driving apparatus of a FED had been proposed(see Korean patent application NO. 95-45457). The cell driving apparatus employs the passive indication method to avoid the complexity of manufacturing processes and achieves an appropriate gray scale by controlling an amount of current provided to a cathode.
The FED disclosed in the above Korean patent application includes a field emission pixel, which contains a cathode and a gate electrode emitting electrons from the cathode, and employs the passive matrix indication method. The cell driving apparatus for use in the FED comprises more than one current source deposited so as to supply a constant current signal to the cathode and a controller selectively operating two or more current sources which generate different amounts of current signals according to the size of a video signal.
The cell driving apparatus of the FED disclosed in the Korean patent application NO. 95-45457 provides various current signals to the cathode by selectively operating two or more current sources according to the size of the video signal to thereby linearly adjust the amount of electrons to be emitted from the cathode. In result, the cell operating apparatus solved the drawbacks due to the lack of uniformity of the tips and the limitation in obtaining the full color.
In the meanwhile, in the cell driving apparatus of the FED disclosed in the Korean patent application NO. 95-45457, high voltage devices were used in designing a current mode DAC such as a current mirror 18, a current valve 20, and a current source 21 which supplies a constant current to the cathode of the FED. The current mode DAC was designed to prevent a high voltage from being instantaneously applied to the cathode, wherein the instantaneous high voltage is due to parasitic capacitance existing on the gate line and the cathode line.
However, since the high voltage MOS device has a lengthily extended drain structure capable of precluding the instantaneous high voltage compared with a low voltage device, it occupies a wide area.
In addition to this, the usage of the high voltage MOS device occupying the wide area can induce a problem when enhancing a gray level represented by a pixel in the FED by minutely dividing a current level which is provided to the cathode since, in order to generate the current having a various level, the current mode DAC should increase the number of components devices thereof.
It is, therefore, a primary object of the present invention to provide a cell driving apparatus of a FED capable of increasing a gray level and minimizing an area problem by designing a current mode DAC which contains low voltage devices.
In accordance with one aspect of the present invention, there is provided a cell driving apparatus for use in a field emission display employing a passive matrix indication method, wherein the field emission display includes a field emission device cell having a cathode and a gate electrode, and a data driving unit outputting digital signals provided from the outside as data signals, comprising: a current mode DAC unit for providing a current to the cathode in response to the data signals from the data driving unit; and a high voltage isolating unit, connected between the current mode DAC unit and a cathode line, for preventing an instantaneous high voltage from being provided to the current mode DAC unit to thereby protect the current mode DAC unit, wherein the instantaneous high voltage is generated between a gate line and the cathode line in response to a gate control signal derived from a gate control unit.
In accordance with another aspect of the present invention, there is provided the cell driving apparatus for use in the FED further comprising a float-preventing unit for precluding the high voltage isolating unit from being floated when the instantaneous high voltage is supplied to the cathode line.